In some internal combustion engines having a plurality of injection valves, it may occur that some of the injection valves are used only relatively infrequently. This is, for example, the case with internal combustion engines that are operated with a variety of fuels, of which one is injected only in a few engine operating states, or in the case of so-called dual fuel injection internal combustion engines that have both injection valves for an intake manifold injection and also injection valves for a direct injection, where some of these injection valves are used only from time to time as a function of the operation.
However, injection valves which are used only at longer time intervals may develop deposits, for example, due to the evaporation of the fuel as a consequence of the waste heat of the internal combustion engine and due to gum formation of the remaining fuel constituents or due to the combustion products generated downstream of the injection valves. Such deposits may lead not only to an impaired operation because of the change in the amounts of injected fuel, but can also result in damage to the injection valves under unfavorable circumstances. Such damage would make it necessary to replace said injection valves. In order to remove the deposits, it is necessary to flush from time to time the infrequently used injection valves with fuel.
DE 10 2008 031 830 A1 has already disclosed a method of the aforementioned kind for operating internal combustion engines, which use a plurality of fuel types and deliver these types of fuel through the various injection valves into the combustion chambers of the cylinders. In order to prevent that under predefined conditions, for example, when one of the injection valves is not constantly injected with fuel or another substance, the subsequent operation of the injection valve is adversely affected due to the soiling of the injection valves with carbon black or other combustion products, this prior art document proposes delivering a first fuel into the cylinders by means of a first injection valve and injecting, at a first engine power output, a second fuel directly into the cylinder by means of a second injection valve, whereas at a second engine power output that is lower than the first engine power output, instead of the second fuel, a flushing substance is injected directly into the cylinder by means of the second injection valve, wherein this flushing substance can comprise the first and/or the second fuel.
If fuel is used to flush the temporarily unused injection valves, the result is, however, a change in the composition of the exhaust gas downstream of the internal combustion engine, because the fuel that is used for the flushing operation causes a decrease in the air-to-fuel ratio in the exhaust gas. This state in turn has a negative impact on the chemical conversion of undesired exhaust gas constituents like hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) in a catalytic converter disposed in the exhaust gas system downstream of the internal combustion engine, because, at least in the case of three-way catalytic converters, an optimal conversion rate in the exhaust gas mandates a constant air-to-fuel mixture in a stoichiometric ratio (λ=1). In principle, it would be conceivable to consider the amount of fuel used for flushing during the pilot control operation of the fuel to air mixture delivered into the cylinder. However, such an approach is very complicated, because, on the one hand, the amount of fuel that is delivered into the injection valves when one or more injection valves are occasionally flushed, can be calculated only with difficulty, for example, because of the formation of carbon black or vapor bubbles in the said injection valves; and, on the other hand, because the computing algorithms that are used in the engine control unit have to be supplemented with additional computing algorithms in order to consider, in the course of the mixture pilot control operation, the amount of additional fuel that is used for flushing as a function of the type of fuel used for flushing and/or as a function of the operating states of the internal combustion engine.
Working on this basis, the object of the invention is to improve a method of the aforementioned kind so that even if the amount of fuel used for flushing is not considered during the mixture pilot control operation, a complete conversion of the undesired exhaust gas constituents in a catalytic converter of an exhaust gas aftertreatment system of the internal combustion engine when the internal combustion engine is running is still guaranteed.